Timing and Mechanism of Ore Precipitation in Porphyry Cu Systems: Insight from LA-ICP-MS Analysis of Fluid Inclusions and In Situ Oxygen Isotope Analysis of Hydrothermal Quartz at Zhunuo Porphyry Cu Deposit, China

The timing and mechanism of ore precipitation in porphyry copper systems are hot topics and remain controversial. The large Miocene collision-related Zhunuo porphyry Cu deposit in the Gangdese belt of southern Tibet, China, was produced by multistage quartz veining and hydrothermal alteration, accompanied by Cu sulfide precipitation. In this study, we have combined cathodoluminescence (CL) petrography with in situ oxygen isotope analysis and fluid inclusion microthermometry and laser ablation-inductively coupled plasma-mass spectrometer microanalysis to constrain the growth history of individual quartz veins, the source and evolution of the hydrothermal fluids, and the timing and mechanism of ore precipitation at Zhunuo. Early quartz A veins associated with potassic alteration are composed of quartz, K-feldspar, biotite, Cu-Fe sulfides, and pyrite. Quartz B veins are composed of quartz, Cu-Fe sulfides, molybdenite, and pyrite. CL imaging shows that quartz grains in the A and B veins consist of abundant early generation of bright-luminescent quartz (Q(A) and Q(B)) with volumetrically minor later generation of dull-luminescent quartz (Q(A-crack) and Q(B-crack)) occurring in the voids or at the margins of the Q(A) and Q(B) veins with embayed contacts. Cu-Fe sulfides are generally in contact with the dull-luminescent quartz and locally in contact with the bright-luminescent quartz and K-feldspar in the A and B veins or occur as disseminations in the potassic-altered porphyries that have been overprinted by chlorite +/- muscovite alteration. Q(A) and Q(B) contain single-phase intermediate-density inclusions and abundant brine and vapor-rich inclusions. A boiling assemblage in Q(A) has a homogenization temperature of similar to 560 degrees C and trapping pressure of similar to 530 bar. Two boiling assemblages in Q(B) have homogenization temperatures of similar to 440 degrees C with trapping pressures of similar to 230 and similar to 250 bar, indicating a transition from lithostatic to hydrostatic conditions at a paleodepth of similar to 2.0 to 2.5 km. Q(A-crack) and Q(B-crack) contain aqueous inclusions with lower homogenization temperatures of 340 degrees to 400 degrees C and salinities of 6 to 12 wt % NaCl equiv. In situ oxygen isotopes indicate that Q(A) and Q(B) have delta O-18 values of 7.6 to 11.4%c, whereas Q(A-crack) and Q(B-crack) have delta O-18 values of -7.2 to 6.7 parts per thousand. Combined with fluid inclusion compositions, we propose that condensation of vapors into brines and mixing with 25 to 60% meteoric water can produce the salinity and oxygen isotopes of fluids that caused the dissolution of the early bright-luminescent quartz and the precipitation of the later dull-luminescent quartz. Zhunuo Cu-Fe sulfides are in contact with the bright-luminescent quartz and cut the growth bands. The dullluminescent quartz in contact with the Cu-Fe sulfides has oscillatory growth banding. In addition, Cu-Fe sulfides in the A and B veins or in the potassic-altered rocks coexist with chlorite +/- muscovite alteration minerals. These geologic observations, together with evidence that there is an abrupt drop in the Cu/(Na + K) ratio by more than one order of magnitude in the aqueous fluids within the dull-luminescent quartz compared to the brines within the bright-luminescent quartz, indicate that precipitation of Cu-Fe sulfides occurred after the potassic alteration stage and during the chlorite +/- muscovite alteration stage. Ore precipitation was associated with mixing of magmatic fluids (brines + vapors) with meteoric water, accompanied by fluid cooling, salinity decrease, and retrograde quartz solubility as the rocks transitioned from lithostatic to hydrostatic conditions. During continued cooling in the latest stage, the hydrothermal fluids precipitated D veins that contain abundant pyrite with minor quartz. Quartz in the D veins (Q(D)) contains aqueous inclusions with much lower temperatures and Cu/(Na + K) ratios, indicating that most of the Cu-Fe sulfides have been precipitated during this stage.